Temperature-dependent thermochemical properties of the Mebicaret (2,4-dimethyl-6,8-diethylglycoluril) solutions in H₂O and D₂O at the ambient pressure

Highlights: • Enthalpies of solution of Mebicaret in H₂O and D₂O were measured calorimetrically. • Enthalpy-isotopic effect was found to be negative and decreasing with temperature. • Standard molar heat capacities of solution are positive and higher in heavy water. • Enthalpic coefficients for pair 2-2-interactions are more negative in heavy water. • The prevailingly hydrophilic hydration of Mebicaret is weakened with temperature. - Abstract: The enthalpies of solution of Mebicaret or 2,4-dimethyl-6,8-diethyl-2,4,6,8-tatraazabicyclo[3.3.0]octane-3,7-dione in ordinary (H₂O) and heavy (D₂O) water at (278.15, 279.15, 288.15, 298.15, 308.15, and 313.15) K as well as the enthalpies of dilution of its H/D isotopically distinguishable aqueous solutions at 298.15 K were measured calorimetrically. The standard (at infinite dilution) molar enthalpies and heat capacities of solution, and the enthalpic coefficients for pair (h₂₂) and triplet (h₂₂₂) interactions between hydrated solute molecules along with D₂O−H₂O solvent isotope effects (IEs) on the studied quantities were computed. The enthalpic effects of Mebicaret dissolution in H₂O and D₂O experience a negative-to-positive sign inversion nearby 303 K and 304 K, respectively, whereas the corresponding IEs were found to be wholly negative (by sign) and decreasing in magnitude with increasing temperature. The h₂₂ values as well as IEs on them were found to be negative, too. On the contrary, the values of h₂₂₂ are positive and comparable in magnitude with h₂₂ ones. These facts indicate that the Mebicaret hydration, being enhanced in the D₂O medium, is dualistic by the nature with the prevailing hydrophilic mechanism.